1. Field of the Invention
This invention relates to synergistic additive combinations which dramatically improve the oxidation stability of lubricating compositions, particularly automatic transmission fluids ("ATF").
2. Description of Related Art
Protection of lubricating compositions from oxidation is an increasingly important function of additive compositions. Merely increasing the treat rate of known conventional antioxidants is frequently insufficient to meet the higher requirements imposed by automobile manufacturers. Thus, there is a continuing search for alternate additives that confer greater oxidation stability to the lubricating composition. This invention is one method toward satisfying this need. We have found that a combination of phosphoric acid with ashless antioxidants, such as alkylated diphenyl amines and hindered phenols, surprisingly increases the oxidation resistance of a lubricating composition compared with either type of component alone.